Vacuum spark gap device having composite electrodes



F. WATERTON 3,102,968

VACUUM SPARK GAP DEVICE HAVING COMPOSITE ELECTRODES Sept. 3, 1963 FiledOct. 26, 1961 United States Patent- O The present invention relates tovacuum spark gap :devices, that is, devices in which a spark gap isdefined within an evacuated enclosure.

Vacuum spark gap devicesfind use, for example, in

surge divertors which comprise a series circuit relationship of one ormore spark gaps and a current limiting resistor connected for protectingan electricity supply a systemso. that damage to the electricalinsulation or apparatus of the systern by excess voltage, as may becaused by a lightning stroke, can be prevented.

Spank gap devices for use in such surge divertors are required to sparkover when a surge voltage exceeds a predetermined value, which isslightly less than a maxpermissible voltage and to fail to spark overwhen the normal voltage of the supply system is applied. .A vacuum sparkgap device has a spark-over or breakdown voltage which is dependent onthe spacing between its electrodes. The curve of break-down voltageplotted against electrode spacing is. not linear, but initially risessteeply with increasing electrode spacing, after which. the curvebecomes asymptotic to some value of breakdown voltage which is largelydetermined by the material from which the electrodes are made- Duetoathesteeprise of this curve over its initial portion, it is difficultto manufacture a vacuum spark gap device having an accurately determinedbreakdown voltage of a desired value if this voltage falls within thatportion of thetcunve, owingto the critical spacing then required forthe-electrodes. Furthermore, with the breakdown voltage lying on the:steep portion of the curve any changes inthe elfective electrodespacing, resulting for instance from the discharge surfaces becomingburnt-away during use; will produce-in consequence a relatively largechange in the breakdown voltage. According to the invention avacuumspark gap device has its gap defined between composite electrodes eachof which comprises a part presenting a reference surface of one metallicmaterial and carrying a 'quantity of another metallic material exposedto-the other elec trode at a position adjacent to said surface,- thesecond material being characteristic of a lower arcing voltage than isthe first material: preferably the material characteristic of the lowerarcing voltage may be contained in one or more cavities in the referencesurface but alternatively or in addition it may be provided as a coatinglayer on the reference surface.

It is preferable that the material used for the reference surface shouldhave a higher melting point than the other material in :order to preventdiffusing of the latter into it during use, it being appreciated thateach time the gap breaks down during use the resultant are between theelectrodes will beat them to a considerable temperature. By way ofexample, the reference surface may be formed from a refractory metalsuch as molybdenum, and the material of lower arcing voltage may be anickel/ tin alloy which has also a lower melting point than molybdenum.

In carrying out the invention one of the materials used for thecomposite electrodes, for example the nickel/tin alloy mentioned, mayalso have gettering properties so that it tends to absorb gases whichmay be produced on arcing.

An advantage of the invention is that by appropriate 3,102,968 .PatentedSept. 3, 1963 choice of the lower arcing voltage material it can 'bearranged that the-voltage rangecovered. by the asymptotic portion of thecurve' of breakdown voltage to electrode spacing includes thedesiredbrea'kdown voltage, -so' that the electrode spacing is lesscritical than it would otherwise be.

Furthermore an are produced between the composite electrodesof a vacuumspark'gap device according to the invention will tendtoroot itself onthe exposed material of lower arcing voltage. ".Eroding action of theare on the reference surfaces of the electrodes Tv'vill therefore bedelayed so that these surfaces will define a gap" of substantiallyconstant dimensions for a relatively longtime.

When the materialof lower arcing voltage is contained in cavities in thereference surface, substantialy the whole of the referencesurfiacetbecomes coated'with this material 'due to vaporisationAan-dsubsequent condensation of the material as a result of arcing. 'I'hecavities provide reservoirs of the material, fromwhich any portions ofthe coat- .ing which are burutolf the reference surface are replaced asaresult'of this vaporisation and subsequent condensation.

. In order that the invention may be more clearly understood referencewill now be made to theaccompanying drawing in which:

FIG. 1 shows in partial section, an example of a vacut um spark gapdevice embodying the invention;

FIG. 2 shows a front end View of a composite electrode shown inelevation in FIG. 1; and

FIG. 3 shows in section, an-altern-ative form of a composite electrodeshown in FIG. 1.

Referring in the first instance to FIGS. 1 and 2, a vacuurn spark gapdevice comprises an insulating envelope 1 which can be of glass or ofporcelain, or other ceramic material, having metal end caps 2 sealed. toeach of its axially opposite ends to form anairatight chamber 4. Two

. conductors 3 which can be of copper or steel, each termin-at'm-g in anelectrode 5, are sealed through respective end caps so that the twoelectrodes 5. project into the chamber 4 from opposite ends andarespaced from each otherto form a spark gap. Communicating with theinterior of the chamber 4 through one of the end caps 2 is a pipe 7 towhich an. evacuating purnpcan be attached and which can be sealed afterevacuation.

.: .The electrodes 5 can. be surrounded by a metal shield 9.to. preventvapour resulting from the action of the arc from being. deposited on theinner wall of the envelope 1,

.the shield 9. being-formed for example, .by an axially splitcylindrical member expanding by .circlips 10 against an :annularabutment 1'. on the inner wall of theienvelope 1. In amodification ofthe embodiment shown in the drawings the envelope 1 maybe of metal andthe conductors 1 sealed through insulating bushings in-the end caps 2.In .thiscase the shield 9 may be dispensedwith. The electrodes 5areiofcomposite metallic construction: each of them defines a referencesurface 6 and carries a quantity of another metallic material which ischaracteristicof a lower arcing voltage than is the material of thereference surface 6. Such a composite metallic construction canconveniently be achieved by forming cavities 8 in the electrodereference surface 6 and filling them with the lower arcing voltagematerial. For example, the body of the electrode 5 can be of arefractory metal such as molybdenum and the cavities 8 can be filledwith a nickel/tin alloy, which may have, for instance, a nickel contentof from 70% to the balance being tin. Alloys of nickel, tin and copperin varying proportions have also been found to be suitable, forinstance, an alloy consisting of 35% nickel, 15% tin and 50% copper, andan alloy consisting of 56% nickel, 24% tin and 20% copper.

Another form of composite electrode shown in FIG. 3, comprises a metalbody 11, for instance of copper, having a number of cavities 8 in itsfront surface and covered on this surface with a conforming layer 12 ofmolybdenum or other refractory metal which also lines the cavities. Thecavities 8 as thus lined are filled with the nickel/tin alloy alreadymentioned as indicated at 13. The forward surfaces 6 of the refractorymetal layer 12 constitute the reference surface previously referred to.The layer 12 may be formed in several ways: for instance it may beelectro-plated on to the body 11 or it may be in the form of a pressedplate of appropriate shape suitably secured on to the body 11, thislatter being preformed with its cavities in both cases, or the layer 12may be constituted by an appropriately shaped pressed plate on to whichthe body 11 is cast.

A further form of composite electrode (not shown) may present simply aplane surface of refractory metal (which forms the reference surface 6)coated with a layer of a metallic characteristic of a lower arcingvoltage than is the material of the reference surface.

In order to prevent the root of an are produced between the electrodes 5from travelling to other parts of the electrode structure, theperipheral side surface of each electrode 5 may be formed with aperipheral cavity (not shown).

Vacuum spark gap devices embodying the invention are eminently suitablefor use in conjunction with current limiting resistors as surgedivertors in electricity supply systems and for the protection of theconductor insulation, transformer windings or the electrical apparatusof the system. For example in a three-phase electricity supply system aspark gap device according to the invention may be connected betweeneach phase conductor and a neutral point through a common currentlimiting resistor which can be linear or non-linear. Alternatively aseries connection of a spark gap and a current limiting resistor can beconnected between each phase conductor and the neutral point. i

Furthermore, the accurately determined breakdown voltage enables vacuumspark gap devices to be used in series to provide arrangements havinglarger breakdown voltages without inaccuracies due to cumulative errors.

Whereas in FIG. 1 the insulating envelope 1 containing the electrodesSis shown as having external rain sheds 14, these may if desired bedispensed with.

\What I claim is:

1. A vacuum spark gap device including two electrodes which define aspark gap between them, each said electrode being a composite structurecomprising a part which presents a reference surface of a first metallicmaterial and a quantity of a second metallic material exposed to theother electrode at a position adjacent to said surface, the secondmaterial being characteristic of a lower arcing voltage than is thefirst material, and the first material having a higher melting pointthan the second material. 7

2. A vacuum spark gap device as claimed in claim 1 including as saidsecond material one which has gettering properties. i

3. A vacuum spark gap device including two electrodes which define aspark gap between them, each said electrode being a composite structurecomprising a part which presents a reference surface of a first metallicmaterial and a coating of a second metallic material provided on saidsurface and exposed to the other electrode the second material beingcharacteristic of a lower arcing voltage than is the first material, andthe first material having a higher melting point than the secondmaterial.

4. A vacuum speak gap device including two electrodes which define aspark gap between them, each said electrode being a composite structurecomprising a part which presents a reference of a first metallicmaterial with at least one cavity defined by said surface, and aquantity of a second metallic material contained in said cavity andexposed to the other electrode, the second material being characteristicof a lower arcing voltage than is the first material, and the firstmaterial having a higher melting point than the second material.

5. A vacuum spark gap device including two electrodes which define aspark gap between them, each said electrode being a composite structurecomprising a reference surface formed by a layer of metallic refractorymaterial and a quantity of another metallic material carried by saidsurface at a position exposed to the other electrode, this othermaterial having a lower melting point than the refractory material ofsaid layer and being characteristic of a lower arcing voltage than thislatter material.

6. A vacuum spark gap device including two electrodes which define aspark gap between them, each said electrode being a composite structurecomprising an electrode body of metallic refractory material whichpresents on a part facing the other electrode a reference surfacedefining at least one cavity, and a quantity of another metallicmaterial in said cavity, the other material having a lower melting pointthan the refractory material of the electrode body and beingcharacteristic of a lower arcing voltage than is this latter material.

7. A vacuum spark gap device including two electnodes which define aspark gap between them, each said electrode being a composite structurecomprising an electrode body having at least one cavity in a surfacefacing the other electrode, a plate of a metallic refractory materialsecured to said surface and shaped to line said cavity, and a quantityof another metallic material contained in the lined cavity, this othermaterial having a lower melting point than the refractory material ofsaid plate and being characteristic of "a lower arcing voltage than isthis latter material.

8 A vacuum spark gap device including two electrodes which define aspark gap between them, each said electrode being a composite structurecomprising an electrode body having at least one cavity in a surfacefacing the other electrode, an electroplated layer of metallicrefractory material on said surface and lining said cavity, and aquantity of another metallic material contained in the lined cavity,this other material having a lower melting point than the refractorymaterial of said layer and being characteristic of a lower arcingvoltage than is this latter material.

No references cited.

1. A VACUUM SPARK GAP DEVICE INCLUDING TWO ELECTRODES WHICH DEFINE ASPARK GAP BETWEEN THEM, EACH SAID ELECTRODE BEING A COMPOSITE STRUCTURECOMPRISING A PART WHICH PRESENTS A REFERENCE SURFACE OF A FIRST METALLICMATERIAL AND A QUANTITY OF A SECOND METALLIC MATERIAL EXPOSED TO THEOTHER ELECTRODE AT A POSITION ADJACENT TO SAID SURFACE, THE SECONDMATERIAL BEING CHARACTERISTIC OF A LOWER ARCING VOLTAGE THAN IS THEFIRST MATERIAL, AND THE FIRST MATERIAL HAVING A HIGHER MELTING POINTTHAN THE SECOND MATERIAL.